Rotary steerable drilling systems for drilling deviated boreholes into the earth are generally classified either as point-the-bit systems or push-the-bit systems. In point-the-bit systems, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly in the general direction of the new portion of the hole being drilled. The borehole is propagated according to customary three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and a lower stabilizer results in a non-collinear condition required for a curve to be generated. In this type of system, the drill bit tends to have less sideways cutting because the bit axis is continually rotated in the direction of the curved borehole.
In push-the-bit rotary steerable systems, there is usually no specially identified mechanism to deviate the bit axis from the local bottom hole assembly axis. Instead, the requisite non-collinear condition is achieved when either upper or lower stabilizers are used to apply an eccentric force or displacement in a direction oriented with respect to the direction of borehole propagation. Steering is again achieved by creating non co-linearity between the drill bit and at least two other touch points. In this type of system, the drill bit is required to cut sideways to generate the desired, curved borehole.
The forces applied to create the non-colinearity and to control the direction of drilling may be provided by a variety of actuators. The actuators provide a tool face oriented to act against a desired component, e.g. against a pivotable sleeve, in a manner that changes or maintains the desired non-collinear orientation of the rotary steerable drilling system. In many applications, difficulties can arise in controlling the actuator tool face in a manner to provide the desired control over the directional drilling.